The present invention relates to an electric motor with permanent-magnet rotor having viscous shaft coupling.
Conventional electric motors having a permanent-magnet rotor comprise a stator, with an electromagnet constituted by a lamination pack and by corresponding windings, and a rotor, which is arranged between two poles formed by the stator and is axially crossed by a shaft which is rotatably coupled to a supporting structure.
It is also well-known that the higher the inertia of the load applied to a synchronous motor, the more difficult it is to start the motor.
Starting in fact occurs as a transient process in which the rotation direction, the speed and the current change until the synchronous state is reached.
During this transient process, the rotor oscillates due to the alternating magnetic field produced by the stator, which by inducing a torque on the permanent-magnet rotor tends to move the rotor into a position in which the magnetic field of the rotor is aligned with the stator field.
If, during this oscillation, the rotor acquires enough kinetic energy to move imperceptibly away from the alignment position, it undergoes a further acceleration which makes it perform another portion of a turn and so forth until the synchronous state is reached.
For an equal power level, the lower the inertia of the applied load, the greater the extent of the oscillations produced on the rotor; accordingly, the rotor is able to accelerate, gaining a speed which allows it to synchronize with the alternating field of the stator.
Viceversa, if the inertia of the load is significant, the extent of the oscillation of the rotor is limited and does not allow to reach the synchronous state.
If the inertia of the load is even greater, the extreme case occurs in which once power has been supplied to the stator the rotor cannot even start the oscillation, i.e., it remains motionless in its equilibrium position.
For load inertias which are not too high with respect to the power level of the motor, couplings of the mechanical type are currently widely used which are inserted between the load and the rotor and allow the rotor, during startup, to oscillate freely through a certain rotation angle (usually 180 sexagesimal degrees).
In this manner, in the startup transient the rotor is disengaged from the inertia of the load and this is advantageous for attaining the synchronous state.
Accordingly, a free rotation occurs through a certain angle, followed by a sudden impact when the load is engaged.
At this point a direct connection between the load and the rotor is obtained; in practice, in operation the two are rigidly coupled.
The mechanical couplings are disclosed in EP 723329, where reference is also made to the application of the motor for a drain pump for a washing machine or dishwasher.
Usually, the inertia represented by the impeller of a pump for such an application is relatively low with respect to the power that can be supplied by the motor. Accordingly, these couplings fully achieve their function, which is indeed to reduce the torque required for startup, giving the correct power rating to the motor with respect to the load that it must drive, providing a consequent benefit to the overall efficiency of the machine and therefore to the cost.
However, there are applications in which the inertia of the load (for example the impeller of a fan) is so great that even the above-cited mechanical coupling is able to start it, unless the motor is oversized so much that it is excessively expensive to manufacture and use, making it accordingly uninteresting for the user.
For these applications, the solution is to provide a system which is able to transmit the torque of the motor gradually to the load during startup.
The aim of the present invention is to provide an electric motor with a permanent-magnet rotor in which gradual traction occurs at startup and in which the rotation rate of the load can be independent of the rotation rate of the rotor.
Within the scope of this aim, a consequent primary object of the present invention is to have a low static torque required to start the permanent-magnet synchronous motor.
Another important object of the present invention is to provide a motor which is constructively simple and compact.
Another important object of the present invention is to provide a motor which is quiet at startup and during operation.
Another object of the present invention is to provide a motor having reduced consumption and a low cost.
This aim, these objects and others which will become apparent hereinafter are achieved by an electric motor with permanent-magnet rotor comprising a stator, with an electromagnet constituted by a lamination pack and associated windings, and a rotor, which is arranged between two poles formed by the stator and is axially crossed by a shaft which is rotatably connected to a supporting structure, characterized in that said rotor is mounted freely on the rotation shaft to which the load is applied and is contained in a hermetic casing which is rigidly coupled to said shaft and contains a working fluid, said rotor and said outer casing being shaped so as to mutually interact only by means of the working fluid, thus allowing smooth variations between the speed of the rotor and the speed of the casing and accordingly between the rotor and the applied load.
Advantageously, the space between the outer surface of the rotor and the inner surface of the casing contains a viscous liquid, so that the rotor, by moving said liquid, turns the casing and therefore the shaft with the load applied thereto.
Conveniently, in a conceptually equivalent different embodiment a bladed impeller is rigidly coupled to at least one of the ends of the rotor and interacts with a corresponding bladed impeller which is rigidly coupled to said casing and is arranged frontally thereto, so as to provide a viscous actuation coupling between the rotor and the casing.